Quantum well infrared photo detector and monolithic chopper

Abstract

A quantum well infrared photodetector and modulator are monolithically integrated to one another. The infrared scene impinges on an infrared receiving element such as a phase grating, which couples it to a modulator. The modulator includes asymmetric quantum wells which are shifted by an applied bias. By appropriately adjusting the bias, the modulator can pass a maximum amount of the input information or can pass less. The photodetector and modulator both use the same kinds of materials, preferably GaAs and Al x Ga 1-x As, so they essentially become a single unit, obtaining better noise and operating characteristics from the combination.

What is claimed is: 
     
       1. An infrared detection system that converts incident infrared radiation indicative of a scene to be imaged into electrical signals comprising: an infrared energy coupling mechanism, formed in a location to receive said incident infrared radiation;   a plurality of modulator layers including first and second materials, formed optically close to said incident infrared energy coupling mechanism, to form a temporal infrared modulator having first and second quantum wells, at least one of which reacts to said incident infrared radiation via intersubband transitions by an amount dependent on an applied bias to effect temporal modulation between a first bias at a first time that allows a first amount of said incident infrared radiation to pass, and a second bias at a second time that allows a second amount of said infrared radiation to pass, said first amount greater than said second amount;   a second plurality of detector layers also including said first and second materials, monolithically integrated with said modulator layers in a location such that said modulator layers are between said infrared energy coupling layers and said detector layers, to form an infrared detector which is monolithically integrated with said temporal infrared modulator layers; and   a controlling element, temporally controlling said bias applied to said modulator layers to varying amounts including said first and second amounts, and thereby chopping said incident infrared radiation to produce a temporally alternating electrical signal.   
     
     
       2. A system as in claim 1 wherein said materials include GaAs and Al x  Ga 1-x  As. 
     
     
       3. A system as in claim 1 wherein said infrared energy coupling mechanism is a phase grating. 
     
     
       4. A system as in claim 1 further comprising a substrate, said substrate including an angled face forming said infrared energy coupling mechanism. 
     
     
       5. A system as in claim 1 further comprising a biasing system, biasing said modulator layers, a first amount of bias forming more absorptive quantum energy wells, and a second amount of bias causing less absorptive quantive energy wells. 
     
     
       6. A system as in claim 5 wherein said modulator layers are formed to maximize absorption amplitude and minimize absorption spectra variation with applied bias. 
     
     
       7. A system as in claim 6 wherein said modulator includes a plurality of barrier layers, a plurality of well layers, and a plurality of coupling barriers. 
     
     
       8. A system as in claim 5 wherein said modulator includes a first narrow well which is depleted by negative bias and populated by positive bias, and a second wider well, wider than said first well, whose absorption does not block a significant portion of radiation in the detector's spectral response range. 
     
     
       9. A system as in claim 1, further comprising: an electronic unit operating to separate an unmodulated DC electrical background signal caused by a dark electrical current present in said detector layers from a total electrical signal detected thereby based on an operation of said temporal infrared modulator layers, to thereby produce an output electrical signal with a reduced contribution from non-radiation induced components present in said infrared detector.   
     
     
       10. A system as in claim 1, further comprising: an electronic unit operating to AC couple said detector layers to reduce noise with a frequency characteristic proportional to 1/f, based on said temporal modulation of said incident infrared radiation by said modulator layers.   
     
     
       11. An infrared modulator system for converting an incident infrared radiation indicative of a scene to be imaged into a temporally alternating electrical signal, comprising a first element with multiple quantum wells, a controlling element and a biasing element, said first element arranged to have multiple quantum wells of an asymmetric type, said controlling element periodically modulating said bias in the time domain and thereby changing absorption of said multiple quantum wells according to said modulating to chop said infrared radiation by altering said absorption of said multiple quantum wells and thereby forming chopped images indicative of said scene to be imaged at discrete and separated times. 
     
     
       12. A system as in claim 11 further comprising a plurality of detector layers, monolithically integrated with said first element such that said first element is between said detector layers and a source of infrared radiation, and to form a modulator/detector array. 
     
     
       13. An infrared detection system for converting an incident infrared radiation indicative of a scene to be imaged into electrical signals, comprising: an infrared energy coupling mechanism, formed to receive infrared energy;   a first plurality of modulator layers, said first plurality of modulator layers including elements for periodically modulating an amount of said incident infrared radiation passing therethrough in the time domain, and having an arrangement to maximize an absorption spectrum variation of intersubband transitions based on a first applied bias;   a first biasing element, selectively biasing said modulator layers via controlling said first applied bias in a first way to maximize said incident infrared radiation passing therethrough, and in a second way to minimize said incident infrared radiation passing therethrough to chop said incident infrared radiation;   a controlling element, periodically modulating said first bias applied to said modulator layers in the time domain and thereby chopping said incident infrared radiation by temporally altering said incident infrared radiation pass therethrough to form chopped images indicative of said scene to be imaged at discrete times; and   a plurality of layers forming an infrared detector, said plurality of layers minimizing variation of absorption spectra with a second applied bias and receiving said chopped infrared radiation from said modulator layers.   
     
     
       14. A system as in claim 13 wherein said modulator layers and said detector layers are formed using common materials. 
     
     
       15. A system as in claim 14 wherein said common materials include GaAs and Al x  Ga 1-x  As. 
     
     
       16. A system as in claim 14 wherein said infrared energy coupling mechanism includes a phase grating. 
     
     
       17. A system as in 13, further comprising: an electronic unit operating to separate an unmodulated DC electrical background signal caused by a dark electrical current present in said infrared detector from a total electrical signal detected thereby based on an operation of said modulator layers, to thereby produce an output electrical signal with a reduced contribution from non-radiation induced components present in said infrared detector.   
     
     
       18. A system as in claim 13, further comprising: an electronic unit operating to AC couple said detector to reduce noise with a frequency characteristic proportional to 1/f, based on said temporal modulation of said incident infrared radiation by said modulator layers.   
     
     
       19. A method of temporally modulating an infrared scene, while maintaining relative spatial radiation intensity distribution thereof unchanged, comprising: providing an infrared modulator mechanism which includes more than one quantum well, each of which captures electromagnetic radiation-produced particles dependent on an applied bias;   applying said bias in a first way to shift the quantum wells in a first asymmetric way to cause the quantum wells each to receive the particles by a first amount and to cause a first amount of energy modulation;   applying said bias in a second way to shift the asymmetric quantum wells to a different relationship which causes them to receive the particles by a different amount than the first amount;   repetitively changing said bias between said first way and said second way in sequence to generate a periodically alternating change in said bias in the time domain, and thereby obtaining a temporally alternating energy modulation on said infrared scene between said first amount and said second amount; and   converting said temporally alternating infrared scene into a temporally alternating signal by a plurality of integrated quantum-well detector layers.   
     
     
       20. A method of detecting an infrared image, comprising: coupling incident radiation from an infrared scene with a spatial radiation modulating element to a temporal radiation modulator that is monolithic integrated to a radiation detector, said temporal radiation modulator and radiation detector having different multiple-quantum-well structures with intersubband transitions;   applying a first bias to said multiple-quantum-well radiation modulator to alter radiation absorbing characteristics thereof;   periodically modulating said incident radiation in the time domain with said multiple-quantum-well radiation modulator by changing said first bias;   applying a second bias to said radiation detector;   separating & background electrical signal caused by a dark electrical current from a detected electrical signal by said radiation detector to produce an output electrical signal with reduced contribution from non-radiation induced components in said radiation detector, thereby to achieve full TV frame rate integration and to prevents saturation of readout circuits;   AC coupling said radiation detector at a frequency substantially identical to said modulating frequency of said radiation modulator to reduce noise with a frequency characteristic proportional to 1/f.   
     
     
       21. A method as in claim 20 wherein said radiation modulator is made of a first multiple-quantum-well structure having confined intersubbands and said first bias applied thereto is set to maximize absorption spectra variation; said radiation detector is made of a second multiple-quantum-well-structure having said second bias applied thereto set to minimize absorption spectra variation. 
     
     
       22. A method as in claim 20 wherein said radiation modulator and said radiation detector are made of substantially identical material compositions including GaAs and Al x  Ga 1-x  As. 
     
     
       23. A method as in claim 20 wherein said spatial radiation modulating element is a phase grating. 
     
     
       24. A method as in claim 20 wherein said spatial radiation modulating element comprises a substrate including an angled facet.